1. Field of the Invention
This invention relates generally to an apparatus and method for use in seismic surveying and more particularly to a seismic source system for use in marine seismic surveying.
2. Discussion of the Prior Art
In marine seismic surveying, to obtain geophysical information relating to the substrata located below the sea bottom, seismic sources, generally acoustic transmitters, adapted to produce pressure pulses or shock waves under water are towed beneath the water surface behind a marine vessel. The shock waves propagate into the substrata beneath the sea where they are refracted and reflected back to the sea. The returning shock waves are detected by sensors (usually hydrophones) and the useful data contained in the signals produced by the sensors is processed to determine the geophysical structure of the substrata.
Air guns or gas guns are frequently used as acoustic transmitters. Usually, several air guns are placed in spaced relation to each other in a subarray. One or more air gun subarrays are towed behind a marine vessel beneath the sea surface. During operation, all air guns in a subarray are activated simultaneously to produce a desired overall pressure pulse from that subarray. The pulse characteristics, such as the frequency, bubble ratio and amplitude, of the overall pressure pulse produced by an air gun subarray is a function of the characteristics of the pressure pulses produced by the individual air guns and the physical arrangement of the air guns in that air gun subarray.
In order to repeatedly produce and transmit pressure pulses having known characteristics under water, it is important that the air gun subarray is maintained at a constant depth below the water surface and in a near straight line horizontal position. Air gun subarrays presently in use are generally more than fifty (50) feet long and weigh several hundred pounds. To tow such an air gun subarray below the water surface, it is a common practice in the art of seismic surveying to pivotly attach a single or multiple floatation devices (buoys) along the length of the air gun subarray by means of a plurality of links. The floatation device maintains the air gun subarray at or near a constant depth below the water surface when the subarray and the floatation device combination (or the seismic source system) is towed behind a vessel.
U.S. Pat. No. 4,686,660 to Gjestrum et al., issued Aug. 11, 1987, discloses one such system which contains a floatation device (buoy) that has several discrete float chambers disposed in longitudinal spaced relation inside a tubular sleeve member. In the alternative, the discrete float chambers may be secured together longitudinally or they may be connectable lengths of a sleeve. Other prior art floatation systems include utilizing discrete float chamber which are either not connected to each other or are serially linked. However, regardless of the manner in which the prior art discrete float chambers are used to form the floatation device, they are not in fluid (generally air) communication with each other and thus, must be pressurized with air prior to use. Once such a floatation system is deployed, no means exist to refill any of the chambers should an air leak occur without shutting down the entire operation and pulling the subarray and the floatation system onto the vessel for repair or replacement. Such prior art pre-airfilled discrete chambers have frequently failed, due largely to minor air leaks in one or more of the discrete chambers over a period of time, causing a portion of the air gun subarray to sag. This sagging distorts the relative positioning of the air guns in the subarray, thereby distortinq the characteristics of the overall pressure pulses produced by that air gun subarray, which, of course, is highly undesirable.
The equipment utilized for seismic surveying includes, among other things, air gun subarrays, seismic cables, data acquisition and processing equipment, and a marine vessel. The total cost of the entire equipment can easily exceed fifteen million dollars ($15,000,000). Due to the high cost of the equipment and the logistics of conducting seismic surveying offshore, the surveying activity is usually, performed around the clock for several days or weeks at a time, except for the time it takes to change crews between working shifts or due to equipment failure. Since there exist no means to replenish the air in the discrete chambers should a leak occur, any failure relating to a floatation system will either require shutting down the surveying activity to pull the floatation system and the air gun on to the vessel or result in obtaining inaccurate seismic data. Neither of these alternatives is, of course, acceptable. It is, therefore, highly desirable to have a reliable floatation system for use in marine seismic surveying whose performance is unaffected by air leaks.
In the prior art, it is typical to tow the air gun subarray and the floatation system from a tow point located in line with the air gun subarray by means of what is commonly known as a "hose bundle." One end of the hose bundle is connected to the tow point and the other is stationed on the deck of the vessel. The hose bundle is pulled by a steel cable attached to it at a suitable place in between the tow point and the vessel. The pulling force on the steel cable is transferred to the hose bundle at the attachment point, making it to be the weak point for the system and thus vulnerable to cracking and breaking.
A typical hose bundle contains a through air hose in the middle for carrying high pressure air to the air gun subarray. The hose is wrapped by one or more layers of electrical conductors for carrying electrical signals between the air guns and sensors on the one hand and the control instrumentation stationed on the vessel on the other hand. Because of the severe bending forces applied to the hose bundle at the attachment point, it tends to damage the air hose and the electrical conductors. It is, therefore, desirable to tow the air gun subarray in a manner which will not damage the hose bundle.
When the subarray is towed from the tow point located on the subarray or which is substantially in line with it, very little or no towing force is applied on the floatation device leaving it free to move in lateral direction. In operation, the air gun subarray is several feet (15-20 feet) below the water surface while the floatation device is at the water surface. Ocean waves at the surface are generally much stronger than at 15 to 20 feet below the surface and cause the floatation device to fish-tail (i.e., cause it to move in a serpentine-like manner), which in turn causes the air gun subarray to also move in the same manner. Again, such a movement of the air gun subarray distorts the relative positioning of the individual air guns in the subarray, distorting the characteristics of the overall pressure pulse. It is, therefore, highly desirable to tow the air gun subarray and the floatation device assembly in a manner which tends to keep both the floatation device and the air gun subarray in straight line when such an assembly is pulled behind a vessel. It is also very desirable to tow an air gun subarray in a manner which does not require towing it by means of a hose bundle. Such a system will require much simpler and less expensive hose bundle construction and improve the overall reliability of the system.
The present invention addresses the above noted problems and provides an air gun subarray and a floatation device assembly (the seismic source system) which utilizes a floatation device that is substantially unaffected by air leaks and which is towed in a manner that tends to keep both the floatation device and the air gun subarray in a straight line horizontal position without imparting substantial tension to the hose bundle.